1. Field of the Invention
The present invention relates to a control method for controlling storage devices in such a way as to maximize their read performance according to a variety of environmental changes, and a retrial method used when data reading from a storage medium fails.
2. Description of the Related Art
With high-density storage of a magnetic disk device, the signal-to-noise ratio (S/N ratio) at the time of data reading has recently degraded. On the other hand, in order to correctly decode a read signal by a read head, an optimal modulation/demodulation method is adopted. Furthermore, the tuning of the parameters of a variety of read circuits, such as a pre-amplifier, the VGA (variable voltage amplifier) circuit of a read channel IC (hereinafter simply called a “read channel”), a filter control circuit and the like, is indispensable depending on the dispersion in characteristics of each read head and a magnetic disk medium.
However, even a device whose parameter is tuned at the time of shipment cannot avoid the change of recording performance due to the change of environmental temperature and the change in the clearance of a read head, and the degradation of a read margin due to output decrease due to the degradation of the read head and the like, which is a problem. In this case, a read margin indicates a probability of successful data reading. The higher the read margin, the lower the probability that an error will occur.
More specifically, when environmental temperature becomes higher or lower than room temperature or when the amount of clearance of a read head changes, the storage performance of a magnetic disk changes. As a result, even when a parameter is tuned before shipment, the read margin degrades in relation to the room temperature, and a read error easily occurs. If too much bias is applied to a read head or if a read head is too old, the performance of the read head degrades. In this case, the head output decreases and the read margin degrades. Therefore, in this case too, a read error easily occurs.
When the storage performance of the magnetic disk device degrades, in order to improve a read margin and to correct an error, an automatic adaptation function that is realized by a variety of filter circuits, signal reshaping circuits and the like that are built in a read channel on a data route is generally used at the time of data reading or read retrial. For an example of a circuit realizing the automatic adaptation function, an FIR (finite impulse response) adaptation control circuit, an asymmetrical correction circuit and the like are used.
In order to cope with output decrease due to the degradation of a read head, an AGC (auto gain control) circuit and the like make a VGA gain follow the current optimal value.
FIG. 1 shows an example of the configuration of a read channel (RDC) with an FIR adaptation control circuit, an asymmetrical correction circuit and an AGC circuit. As shown in FIG. 1, a read channel 100 comprises a VGA (variable voltage amplifier) circuit 101, an asymmetrical correction circuit 102, a CFT (continuous time filter) circuit 103, an A/D (analog/digital) conversion circuit 104, an FIR circuit 105, a Viterbi detection circuit 106, a post-processor circuit 107, a decoder 108, a NRZ interface 109, an FIR adaptation control 110, a PLL (phase-locked loop) circuit 111, an asymmetrical correction control circuit 112 and an AGC circuit 113. Although in FIG. 1, a read channel adopting a PRML (partial response maximum likelihood) method is used as an example, this does not mean to restrict a method adopted by a read channel. In FIG. 1, although a Viterbi detector is used as an error correction circuit, another error correction circuit can also be used.
A pre-amplifier, which is not shown in FIG. 1, outputs an analog signal that is read from the storage medium of the magnetic disk device by a read head, to the VGA circuit 101.
The VGA circuit 101 amplifies an analog signal inputted from the pre-amplifier based on the output from the AGC circuit 113 and outputs it to the asymmetrical correction circuit 102. The gain of the VGA circuit 101 varies depending on the read head that is used to read data or whether a sector to be read is located within the inner circumference or outside the outer circumference of the magnetic disk.
The asymmetrical correction circuit 102 reshapes the waveform of the analog sign and outputs the reshaped signal to the CTF circuit 103 if there is a vertical asymmetry in the waveform. The CTF circuit 103 filters the inputted analog signal. The analog signal filtered by the CTF circuit 103 is converted into a digital signal by the A/D conversion circuit 104.
The FIR circuit 105 filters the inputted digital signal. The Viterbi detection circuit 106 obtains the most probable digital signal from the digital signals equalized by the FIR circuit 105. Then, demodulated data is obtained by demodulating the digital signal by the post processor circuit 107 and the decoder circuit 108. The demodulated data is outputted to a hard disk controller (HDC) through the NRZ interface 109.
Generally, a variety of parameters of the pre-amplifier, the VGA circuit 101, the asymmetrical correction circuit 102 the asymmetrical correction circuit 103, the A/D conversion circuit 104, and the FIR circuit 105 are optimized at the time of shipment. However, since each of the read margins of these circuits varies depending on the change of environmental temperature after shipment, head degradation and the like, the following circuits sometimes make the respective parameters of each read circuit and filter circuit follow the current optimal value.
The FIR adaptation control circuit 110 makes the filtering parameter of the FIR circuit 105 follow the current optimal value, based on the difference in output between the FIR circuit 105 and the Viterbi detection circuit 106.
The PLL circuit 111 makes the parameter for controlling the frequency of the output from the A/D conversion circuit 104 follow the current optimal value, based on the output from the error detection circuit 106.
The symmetrical correction control circuit 112 makes the parameter of the asymmetrical correction circuit 102 follow the current optimal value, based on the digital signal inputted from the FIR circuit 105.
The AGC circuit 113 compares the level of the digital signal inputted from the FIR circuit 105 with a prescribed level, and adjusts the gain of the VGA circuit 101 in such a way that the level of the signal inputted from the pre-amplifier becomes a desired value.
The RDC 100 can also be provided with a circuit making the parameter of the CTF circuit 103 follow an optimal value and the like, which are not shown in FIG. 1.
A conventional error correction method is described below with reference to FIG. 2. In FIG. 2, the horizontal axis indicates time. The upper section of FIG. 2 indicates an index pulse. An index pulse is outputted every time a magnetic disk makes one rotation, and indicates the number of rotations of the disk. The lower section of FIG. 2 indicates the assertion/negation of a read gate, that is, the opening/closing of a read gate.
As shown in FIG. 2, if the reading of the information written in a specific section fails, conventionally, the disk is rotated again at least once, and the read gate is asserted every time its read head reaches a sector position where the data that the read head has failed to read can be read. While the read gate is asserted, the read channel enters into a read mode, and a one time re-reading (read retrial) of the sector that has not been read is conducted. In this read retrial, each parameter of the read circuits and filter circuits that constitute a read channel is made to follow the current optimal value using a circuit realizing the auto follow-up function. If the n-th (“n” is an arbitrary natural number) read retrial fails, the disk is rotated at least one more time and the n+1-th read retrial is conducted. In the n+1-th read retrial, the follow-up operation is started from a parameter that is made to follow the current optimal value in the n-th read retrial.
By bringing each of the parameters of the read circuits and the filter circuits close to the current optimal value in this way, the error of a sector that has failed to be read is corrected. When the number of retrials or time spent in the retrial processes reaches an upper limit, the error cannot be corrected (retrial-out).
Time needed to correct an error is described below with reference to FIG. 3. In FIG. 3, the horizontal and vertical axes indicate the number of retrials and a read margin, respectively. A dotted line indicates a level in which the probability that a read error will occur can be almost neglected. When a read margin goes beyond the dotted line, it means an error has been corrected. In other words, the higher the read margin, the lower the probability that an error will occur.
As shown in FIG. 3, the lower the read margin at the time of error occurrence, the greater the number of read retrials needed to complete an error correction. Specifically, three cases, A, B and C, shown in FIG. 3, are described below. In case A, where the read margin at the time of error occurrence is the highest, in other words, the difference between each current parameter of each read circuit and the current optimal value is not so big, an error correction is completed by one retrial. However, in case C, where the read margin at the time of error occurrence is the lowest, in other words, the difference between the current parameter of each read circuit and the current optimal value is big, six retrials are needed to complete the error correction.
Conventionally, a problem exists where a relatively long time is needed to make the parameter of each read circuit that constitutes a read channel follow the current optimal value using a circuit realizing the automatic adaptation function when the storage performance of a magnetic disk device degrades due to the change of environmental temperature or the like. Therefore, correct data read cannot be expected before a readable state is reached by making the parameter follow the current optimal value, and as a result, the number of read retrials is large. This means the degradation of the performance of the magnetic disk device. In the worst case, it also is the cause of the problem that an error could not be corrected even after the number of retrials or the processing time has reached the upper limit.
If there is a big difference between a tuning value and a current optimal value (that is, if there is big output decrease), there is also a problem where it takes a relatively long time to make a value obtained by tuning a VGA gain to follow the current optimal value before shipment using an AGC circuit and the like in order to cope with the decrease of head output due to the degradation of a read head. Therefore, in this case too, there is a problem where the number of read retrials became large or an error cannot be corrected.
The problems described above are not limited to the case where the storage medium is a magnetic disk, and may occur in the case where the storage medium is another type of disk, such as an optical disk, a magneto-optical disk, a magnetic tape or the like.